Viscose rayon was the original precursor material employed in the manufacture of large production quantities of carbon fibers. The carbon fibers formed from rayon precursor fibers were used for many purposes including the reinforcement of composite material used for ablative and insulative components exposed to very high temperatures accompanied by highly erosive gas or airflow conditions, such as in the nozzle assembly of a solid propellent rocket motor or for a nose cone of a re-entry vehicle. Indeed, the early design criteria for these components were based on the use of carbon fibers specifically derived from viscose rayon.
As time went by, the use of rayon as a precursor for carbon fibers was largely supplanted by carbon fibers derived from polyacrylonitrile (PAN) precursor, except in two very important areas which still depend on rayon as a precursor for the required carbon fibers. These are the ablative thermal protection components used for the re-entry nose for the NASA space shuttle, and the solid propellant rocket motor nozzle assemblies both for space launch and DOD tactical weapons systems. Approximately twelve thousand pounds of carbon fibers are used in every NASA space shuttle launch.
In almost every other application a conversion has been made to the PAN precursor carbon fiber. Carbon fibers produced from PAN are not only considerably less expensive than carbon fibers produced from rayon, but they have substantially improved properties and performance characteristics in most regards, such as excellent tensile strength which is needed for most uses of carbon fiber reinforcing material. On the other hand, the conventional PAN precursor carbon fiber has a much higher thermal conductivity, and consequently does not function as a satisfactory thermal insulator and therefore cannot be used as a replacement for rayon based carbon fiber where low thermal conductivity properties are most important.
The thermal conductivity is in reference to the fabricated insulator component which contains the carbon fiber. Rocket motor test firings conducted by U.S. agencies have shown that insulators containing PAN based carbon fibers develop a significantly deeper char layer as compared to insulators using rayon based carbon fibers. Thus, the use of PAN based carbon fibers in fabricated solid propellant rocket motor insulators greatly increases the difficult thermal management problems associated with generated propellant burnt gases exceeding 5,000.degree. F. Also of great importance is the approximate 20% difference in specific gravity between the PAN and rayon based carbon fibers. The use of PAN based fibers in place of the rayon based fibers would result in a substantial increase in weight for a satisfactory functional insulator which is contrary to what is desired. Therefore, NASA is not willing to take the risk of changing to PAN, especially in consideration of the Challenger disaster.
However, the continued use of rayon as a precursor of carbon fibers has other problems. First, the rayon used to produce carbon fibers for this purpose must be of very high purity, and the cost of carbon fibers made from higher purity rayon is at present approximately $50/lb. compared with a cost of PAN precursor carbon fibers of only about $26/lb. Even more importantly, however, is the fact that the production of such high purity rayon fibers causes substantial pollution. In recent years these fibers have been produced at only one factory in Virginia, and in the past few months this factory has finally been closed as a pollution offender, and so there is no longer an approved domestic source for high purity rayon which can be converted into the desired carbon fiber. While NASA has a store of acceptable material based on the rayon fibers to last for some time, eventually either a new rayon source or a replacement material will be needed. Currently there is a "crash" program supported by NASA and DOD to qualify another domestic rayon fiber producer as a source for the carbon fiber precursor. This is only a stop gap measure being used to buy time until a satisfactory replacement for the rayon precursor carbon fiber can be developed.
A number of years ago some tests were conducted with a solid propellent rocket motor using a nozzle assembly in which PAN based carbon fibers had been stretch broken and spun into yarn prior to nozzle fabrication. These tests showed about a 30% reduction in thermal conductivity along the lengths of the fiber compared with conventional PAN-based carbon fibers, but no change across the width or diameter of the fiber yarn. The added costs were substantial. It was decided that the improvement provided by this technique was insufficient bearing in mind the added costs which were unacceptable, and the project was abandoned.
The patent literature (see U.S. Pat. Nos. 3,841,079 and 3,925,524) show that Celanese Corporation has experimented with the post-treatment of PAN fibers to produce what is referred to as a "microporous" carbon filament. As explained in Kimmel et al U.S. Pat. No. 3,925,524, the post-treatment of the dry spun PAN filament involves contacting the filament with water. It is well known that PAN absorbs water and indeed it is even known according to the literature that water acts as a blowing agent for PAN, and steps are invariably taken to avoid the presence of water in the PAN spinning solution. For example, the BP Chemicals International Technical Bulletin on their "Barex 210" PAN Resin (Brochure B210-05), copy attached and made a part hereof, states that the resin should be pre-dried before extrusion. This brochure moreover states:
Resin Handling-Drying
As with most nitrile copolymers, Barex 210 resin is hydroscopic and will absorb up to 0.3% moisture in an 8 hour period in hot, humid conditions. If processed at this moisture levels, bubbles will appear in the extruded web. PA1 Barex 210 resin is shipped predried . . . . However, some drying is required for regrind. Information on driers, temperatures, residence time, etc., are contained in the bulletin entitled "Drying Characteristics of Barex 210 Resin". Conveying equipment using ambient air is normally used for transferring from the drier to the extruder hopper. However, long conveying runs with ambient air should be avoided to keep the moisture level of resin low. PA1 Barex 210 resin processes much better when care is taken to keep it dry, as is the case with most resins. If proper attention is not given to maintaining a low moisture level, a "shark skin" surface or low melt viscosity and melt strength can result. A "sticky" condition can also occur, and bubbles or foaming will result in the most severe cases.
In their brochure B210-04 entitled "Drying Characteristics", copy attached and made a part hereof, BP Chemicals International states:
Returning now to the work of Celanese Corporation, a review of the '524 and '079 patents does not suggest any deviation from the accepted practice in the art that the resin must be kept dry during formation of the filaments from PAN. The post treatment operations of Celanese as set forth in the Kimmel et al '524 and the Ram et al '079 patents is apparently capable of reducing the specific gravity only very slightly. Thus, according to the '524 patent (column 10, lines 28 and 68), it was possible to reduce the specific gravity to about 1.63-1.65, the typical PAN based carbon fibers having a specific gravity of about 1.7-1.9. According to the Ram et al U.S. patent '079 (see column 12, line 41), the specific gravity was reduced to only 1.75. The resultant carbon fibers retain good strength properties.
The manufacture of porous and/or cellular polymer filaments of various types, such as for textile uses, is of course well known. For example, the patents of Li et al U.S. Pat. Nos. 4,753,762 and Oppenlander 3,422,171 both show methods for producing foamed polymer filaments. Thus Oppenlander '171 discloses the manufacture of fine denier (5-18 mils), foamed and oriented polypropylene monofilaments. Li et al show the formation of closed-cell foamed fibers formed of siloxane polymers, polyesters, polyamides, etc. Microcellular paintbrush bristles of synthetic polymers have recently also been disclosed to the public. Insofar as is known, however, the manufacture of carbon fibers from such foamed filaments has never been contemplated or seriously considered, perhaps because for most uses carbon fibers must have good tensile strength, whereas it is known that foamed plastic has a markedly reduced tensile strength compared to non-foamed plastic.
Lastly, hollow fibers are also known. For example, hollow paintbrush bristles are known from the patent to Ward et al U.S. Pat. No. 4,307,478. Hollow synthetic fibers for other purposes are also known, e.g. Dupont has on the market a hollow filament for insulation purposes sold under the trademark "Hollow-Fil". Again, insofar as is known, it has never been contemplated to convert hollow synthetic filaments of any kind to carbon filaments.